Allow multiple providers in Planner DSL

[simonmcconnell]

I wanted to have multiple providers for a specific config sectionin a Planner:

  config :endpoint, [
    file("config.toml", [
      {:port, ["something", "port"]}
    ]),
    env([
      {:something_else, "SOMETHING_ELSE"}
    ])
  ]

I also added .editorconfig and .gitattributes to force LF newlines so that tests can be run on Windows machines.

Thoughts?